Cosmic dust analyzer

ABSTRACT

Methods and apparatus are provided which employ ion time-offlight techniques to determine the constituency of a high velocity particle of matter such as a micrometeorite. A charged target electrode formed of two known materials is arranged to intercept the particle, the impact of which will create a discrete clous or plasma of ions of both the known target material as well as the particle matter. A charged collector electrode is spaced a preselected distance from the target electrode to receive the ions. However, the region between the two electrodes is established as a low density field-free region, and if the ions are then accelerated into the collector electrode at a proper body, the ions in the plasma will tend to travel at a velocity which is substantially a function of only the mass of the ion. The ions in the plasma will tend to separate into groups according to mass. The fractional ionization for an arbitrary atomic species can be specified by the Saha equation if the plasma volume (V) and temperature (T) are known. T can be determined by taking the ratio of the Saha equations for two elements present in the target in known concentration. (Taking the ratio negates the requirement of knowing V.) Given T, the procedure can be reversed to yield the relative abundance of elements contained in the impacting particle.

United States Patent Fletcher et a1.

COSMIC DUST ANALYZER Inventors: James C. Fletcher, Administrator of theNational Aeronautics and Space Administration, with respect to aninvention of Neal L. Roy, Redondo Beach, Calif.

Filed: May 31, 1974 Appl. No.: 475,338

Related US. Application Data Continuation-impart of Ser. No. 189,438,Oct. 14,

1971, abandoned.

[52] US. Cl. 250/251; 250/287; 250/423 [51] Int. Cl. H01J 39/34 [58]Field of Search 250/423, 424, 281, 282, 250/283, 287, 288, 251; 324/71CP [56] References Cited UNITED STATES PATENTS 3,538,328 11/1970Strausser 250 OTHER PUBLICATIONS Detection Technique for Micrometeoroidsusing Impact Ionization, Auer et al., Earth and Planetary ScienceLetters, Vol. 4, No. 2., Apr., 1968.

Primary Examiner-Craig E. Church Attorney, Agent, or Firm-Russell E.Schlorff; John R. Manning; Marvin F. Matthews [5 7 ABSTRACT Methods andapparatus are provided which employ ion time-of-flight techniques todetermine the constituency of a high velocity particle of matter such asa micrometeorite. A charged target electrode formed of two knownmaterials is arranged to intercept the particle, the impact of whichwill create a discrete clous or plasma of ions of both the known targetmaterial as well as the particle matter. A charged collector electrodeis spaced a preselected distance from the target electrode to receivethe ions. However, the region between the two electrodes is establishedas a low density field-free region, and if the ions are then acceleratedinto the collector electrode at a proper body, the ions in the plasmawill tend to travel at a velocity which is substantially a function ofonly the mass of the ion. The ions in the plasma will tend to separateinto groups according to mass. The fractional ionization for anarbitrary atomic species can be specified by the Saha equation if theplasma volume (V) and temperature (T) are known. T can be determined bytaking the ratio of the Saha equations for two elements present in thetarget in known concentration. (Taking the ratio negates the requirementof knowing V.) Given T, the procedure can be reversed to yield therelative abundance of elements contained in the impacting particle.

11 Claims, 11 Drawing Figures l PAR 1 1 10 W3 1 /ON FLIGHT 2 1 PATH +V W2 I 6 T g L 7 I 2 I 1 a 7 f l f 9 osc/uos COPE TO EXT TO DISPLAY TRIGGERU.S. Patent Oct. 28, 1975 Sheet 1 of9 3,916,187

FIG. I

4 L7 L- IO F3 I ION FL/GHT 2 PATH 9 +V VA A I 6 OSC/LLOSCOPE TO EXT TOOIsPLAV TRIGGER 44 HIGH VOLTAGE SUPPLW-ZO KV) j 43 51 i 48 HIGH VOLTAGETo I SUPPLY IO KV 42 h OSCILLOSCOPE i VERTICAL I 49 AMPLIFIER 40/]? 43Al '53 J 40 i 54 45/ LIGHT PHOTO J/ PIPE i i MULTlPL/ER 50 I 46 I I 43A47 PMT HIGH VOLTAGE I 52/ SUPPLY g 43 i 48 *l T0 cOA COLLECTOR AMPLIFIERUS. Patent Oct. 28, 1975 Sheet 5 of9 3,916,187

Rwmmmwww U.S. Pat ent Oct. 28, 1975 Shee't 6 of9 3,916,187

m mlk T I I l l l l l l l J qmh US. Patent Oct. 28, 1975 Sheet 7 of 93,916,187

Sheet 8 of 9 US. Patent Oct. 28, 1975 FIGQB COSMIC DUST ANALYZER ORIGINOF THE INVENTION The invention described herein was made in theperformance of work under a NASA contract and is subject to theprovisions of Section 305 of the National Aeronautics and Space Act of1958, Public Law 85-568 (72 Stat. 435; 45 U.S.C. 2457) The presentapplication is a Continuation-In-Part application of U.S. Pat.Application Ser. No.: 189,438, filed Oct. 14, 1971, entitled: COSMICDUST ANALY- ZER (now abandoned).

BACKGROUND OF THE INVENTION This invention relatesto improved massspectrometry methods and apparatus and, more particularly, relates tomass spectrometry methods and apparatus employing impactionizationtime-of-flight techniques for investigating the elemental compositionconstituency of cosmic dust particles of celestial matter. I

It is now generally understood that the region which is commonlyreferred to as outer space," and which is the area outside of the earthsatmosphere, is not at:-

vtually a space in the sense of an empty void. As a distributedthroughout space. This material, which is" technically identified asmicrometeorites and which is more commonly referred to as cosmic dust,is fartoo small to be seen by the most'powerful telescope. Nevertheless, there is considerable interest in the nature and origin ofcosmic dust, since there is reason to believe that a knowledge .of theorigin of this material maybe the key to a knowledge of the origin ofourearth.

A particle of cosmic dust is far too small to be captured in the mannerthat U.S. astronauts have taken samples of the moon. As a matter offact, many such These and other disadvantages of the prior art areovercome with the present invention, however, andnovel methods andapparatus are herewith provided which produce a detection signal whichis an unambiguous identification of the signature of the detectedparticle.

SUMMARY OF THE INVENTION In an optimum embodiment of the presentinvention, mass spectrometry apparatus is preferably provided whichutilizes the ionization produced by the impact of a hyper-velocityparticle upon a solid target of suitable design. The impact tends toresult in the creation of micro-plasma which is composed of ions oftarget material as well as ions of the micrometeorite material.Accordingly, means is preferably employed which utilizes iontime-of-flight techniques to separate the microplasma into itsconstituent parts.

In particular, the ions of micro-plasma are preferably acceleratedthrough a predetermined voltage gradient and allowed to drift in afield-free region to a collector electrode. During such drift, however,the ions tend to separate according to mass, and, since the mass of thetarget ions is known, the ions of micrometeorite material can beidentified according to transit time. Moreover, means is preferablyincluded whereby the number of ions of each species may be counted todetermine the relative abundance of each'element so identified in eachimpacting particle ofcosmic dust.

As will hereinafter be explained in detail, the transit time measurementfor each ion in the micro-plasma produced by an impacting dust particleis preferably made by collecting these ions on a flat plate whichgenerates a functionally related signal. The collector plate ispreferably connected to suitable electronics whereby this signal may beappropriately amplified and displayed so that it increases in astep-wise manner after each such impact on the target. The resultingsteps are thenidentifiable as to atomic mass, and the magnitude of eachwill determine the number of ions of a particular'spe cies. Accordingly,the characteristic shape of the collector signal will provide anunambiguous signature of each micrometeorite which impacts on thetarget. If

cosmic dust particles'which may be of special interest would actually betoo small to be seen even close-up," so to speak. For these and otherequally obvious reasons, there has heretofore been no possible way todetermine the constituency of micrometeorites notwithstanding thelongstanding curiosity about this ma terial which has existed inastronomical circles.

It is well known that methods are now available for orbiting packages oftest gear about'the earth at locations outside the earths atmosphere. Ithas been suggested that since these dust particles can'never penetratethe earthsiatrnosphere without being instantly incinerated, suitabletest gear be included in one or more of these satellites which areorbited about the earth in this manner. Moreover, it has been proposedthat the proper kind of test gear to analyze the constituency ofmicrometeorite matter is a mass spectrometer. However, the conventionalmass spectrometer has not been found to be suitable for this purpose, byreason that this apparatus of the prior art tends to producespurioussignals on many occasions.

the impact target contains two elements in known concentration,measurement of the relative ion signals of these two elements can bereduced to plasma temperature by taking the ratio of the Saha equation.Given the temperature, the further theoretical relationships forprocessing the data to yield the relative atomic abundances of theelemental constituents in the impacting particle are provided.

These and other features and advantages of the prior art will becomeapparent from the following detailed description, wherein reference ismade to the figures of the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS Reference to the drawings will furtherexplain the invention wherein like numerals refer to like parts, and inwhich: I

FIG. 1 is a simplified functional diagram of a time-offlight massspectrometer which is suitable for analyzing the constituency of aparticle of cosmic dust or the like.

FIG. 2 is a functional representation of one form of mass spectrometeremploying the concept of the present invention and especially suitablefor use in an ambient life-sustaining environment.

FIG. 3 is a functional representation of another form of massspectrometer of the type illustrated generally in FIG. 2 but employing adifferent collector electrode assembly.

FIG. 4 is a more detailed but nevertheless functional representation ofthe collector electrode assembly hereinbefore referred to in connectionwith FIG. 3.

FIG. 5 is a functional diagram of the major components of a massspectrometer and registration circuit which is suitable for use in outerspace.

FIG. 6 is a schematic diagram of one portion of the apparatus referredto in general in FIG. 5.

FIG. 7 is a schematic diagram of another portion of the apparatusreferred to in general in FIG. 5.

FIG. 8 is a schematic diagram of another portion of the apparatusreferred to in general in FIG. 5.

FIG. 9 is a schematic diagram of another portion of the apparatusreferred to in general in FIG. 5.

FIG. 10 is a pictorial representation of another embodiment of thepresent invention which is suitable for use in conventionalenvironments.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. 1, theremay be seen a simplified sketch of the basic components of a massspectrometer suitable for use in analyzing particles of cosmic dust. Inthis apparatus, a suitable metallic impact plate 2 is arranged toreceive the dust particle which may approach along a path indicated bythe heavy black arrow 10. The impact plate is preferably spaced apreselected distance L, from an accelerating grid 4 which, in turn, isspaced a preselected distance L from an ion collector electrode 3.

The grid 4 is preferably coupled to reference voltage (hereinafterdenoted as ground), whereas the impact plate 2 is preferably coupledthrough a resistance 7 to a positive voltage supply. The impact plate 2will, therefore, preferably be maintained at a preselected positivepotential relative to grid 4, which will hereinafter be referred to as+V The collector electrode 3 is coupled to ground by way of a secondresistance 5 of preselected magnitude, whereby the collector electrode 3will be maintained at a preselected positive voltage relative to thegrid 4 but at a preselected negative voltage relative to the impactplate 2.

As further indicated in FIG. 1, an appropriate oscilloscope circuit 9 ispreferably included for the purpose of providing an observableindication of both the arrival of a particle of cosmic dust and thetime-of-flight of the various constituent parts of the plasma which iscreated by the impact of the particle on the impact plate 2. Moreparticularly, however, the trigger side of the oscilloscope 9 is coupledto the output of an amplifier 8 having its input side coupled to theimpact plate 2. The display side of the oscilloscope 9 is, therefore,coupled to the output of another amplifier 6 having its inputcoupled tothe ion collector electrode 3.

Referring again to FIG. 1, it may be seen that dust particles to beanalyzed will approach and strike the impact plate 2 preferably alongthe path indicated by the arrow 10. Upon impacting the surface of theplate 2, however, the high velocity particle will produce a cloud ofimpact plasma which is composed of ions of target material as well asions of the impacting dust particle.

The cloud of impact plasma will immediately begin to expand uponcreation, and, as the plasma approaches the collisionless state, theelectrons will separate from the cloud as a result of the electric fieldbetween the grid 4 and the plate 2 and will return to the plate 2. Theions, however, will be accelerated toward the grid 4 and along arrow 11at a velocity which may be stated as:

,u ZeV Accordingly, it will be apparent that a measurement of thetransit time of an ion group will uniquely define the charge-to-massratio of the ions in the group. Assuming that the atoms will all besingly ionized, which is most likely because of the relatively lowplasma temperatures involved, a measurement of the ion time-offlightwill define the atomic mass of the ion and will thus identify theelement.

As hereinbefore stated, the collection of electrons at the impact plate2 will coincide essentially with the time of impact of the particle, andthis will produce the start" pulse which is applied to the trigger ofthe oscilloscope 9. The electron flow through the resistor 7 developsthe voltage pulse required to start the sweep of the recordingoscilloscope 9.

The ion collector amplifier 6 may be either current sensitive orcharge-sensitive. In the current-sensitive mode, a series of peaks ofvarying amplitude and time locations may be observed which correspond toions of different elements. In the integrated or charge-sensitive mode,however, the displayed signal tends to increase in steps to its maximumvalue and thereafter to decay according to a particular time-constant oflong duration. Thus, the leading edge of each step will be functionallyrelated to the arrival time of an ion group, and the amplitude of eachstep will be a function of the total charge associated with thatparticular ion group. Hence, the charge-sensitive mode is preferred fortest apparatus intended to be employed in outer space, because the useof such a mode will clearly reduce the complexity of the variouscircuits which must be provided with a cosmic dust analyzer of thischaracter.

Referring now to FIG. 10, there may be seen a simplified pictorialrepresentation of the overall configuration of a particle analyzer whichembodies the concept of the present invention but which is useful undermore conventional circumstances (i.e., in ambient atmospheres which arecapable of sustaining life). Accordingly, there may be seen atime-of-flight chamber which is adapted to support an ion collectorelectrode (not depicted) in one end and an impact plate or target (notdepicted) at the other end. The end of the chamber 90 containing thetarget is preferably provided with a suitable fitting 91 for connectingthe interior of the chamber 90 to an appropriate vacuum pump (sug gestedbut not depicted). The other end of the chamber 90 is coupled to meansfor appropriately introducing a particle to be analyzed into the chamber90.

More particularly, the particle introduction or acceleration assemblymay be seen to be composed of a 2MV Van deGraaff generator 70, or thelike, which has preferably been modified for acceleration of particleson the order of one micron in size. Also provided are suitable particleinjection electronics 71, for electrically charging and introducing theparticle to the Van deGraaff generator 70, and suitable controlelectronics- 72 for the genrator 70.

Particles exiting from the Van deGraaff generator 70 are transmittedthrough a column assembly 74 to the chamber 90. The first stage of thecolumn assembly 74 is a suitable T-joint 75 for coupling the interior ofthe column assembly 74 to an auxiliary vacuum pump (not depicted) ofsuitable design. From there, however, the particle travels through anappropriate magnet assembly 76 which removes unwanted ions from thebeam" which may have been created by the particle charging process inthe generator 70. From there, the particle will traverse a particleposition detector 77 which cates the particle beam axis in order toalign the particle with the system, as will hereinafter be explained indetail.

The next two stages of the column 74, which are traversed by theparticle, are two transit time detectors 78 and 80 which are spaced apreselected distance and which generate timing signals whereby thetransit time or velocity of the particle in question may be determined.Accordingly these timing signals are appropriately coupled to a timeinterval selector and dual proportional delay generator 79. This unithas two principal functions. First, it provides an output pulse 79A tothe electronics 81 of a particle deflector circuit 82 whenever themeasured time interval as indicated by the pulses from the detectors 78and 80 falls within the limits of a predetermined time interval.Normally, the electronics 81 generates a constant input signal to theparticle deflector circuit 82, which conditions the deflector circuit 82to deflect all incoming particles by bias voltage on a pair of deflectorplates (not depicted) and to prevent such particles from continuingthrough the column 74 to the chamber 90. When a particle traverses thetransit time detectors 78 and 80 at a proper velocity, however, thegenerator 79 will generate a signal 79A which conditions the electronics8] to remove the blocking signal from the particle deflector circuit 82,whereby the particle to be tested is permitted to proceed through thecolumn 74 to the chamber 90.

The other purpose of the generator 79 is to produce two trigger pulses79B and 79C at adjustable multiples of the actual transit time asdetermined by the interval between the timing pulses generated by thetwo transit time detectors 78 and 80. By selecting appropriatemultiplication factors, these two pulses 79B and 79C can be made toappear when the approved particle arrives at two arbitrarily selecteddownstream locations which are indicated in FIG. 10 as X and X These twopulses 79B and 79C may be employed to initiate the sweep of theoscilloscope in the display and recording equipment 94, and thus thefirst pulse 798 will preferably be set to occur just before the particleenters the particle charge detector 84 at location X and the secondpulse 79C will preferably be generated when the particlesubsequentlyfarrives at a location just in front of the target in thechamber 90 and indicated in FIG. 10 as X Referring again to FIG. 10, itwill be noted that the column 83 is positioned between the deflector 82and the particle position indicator 84, and which is suitable forconnectingthe interior of the column 74 to an auxiliary vacuum pump(suggested but not depicted). In addition a camera 95 of appropriatedesign may be coupled to the display equipment 94 to obtain a permanentrecord of the images which appear on the oscilloscope in the displayequipment 94.

Referring again to FIG. 10, it will be seen that a particle which exitsthe position detector 84 must first cross position X whereupon thegenerator 79' generates an appropriate actuating signal 79B to cycle thedisplay equipment 94, then must traverse the sensitive particle detector85 before entering the chamber 90, whereupon a signal is generated whoseamplitude and transit time is presented to the camera 95 from whichparticle 1 parameters may be calculated. Upon entering the chamber 90,however, the particle will first pass the ion collector plate orelectrode before impacting on the target, as hereinbefore explained.Accordingly, the chamber is preferably provided with suitableelectronics 93 for registering the mass and number of ions which aregenerated by such impact. Other control electronics 96 will preferablybe included, as will hereinafter be explained.

Referring now to FIG. 2, there may be seen a functional representationof a mass spectrometer of the type depicted generally in FIG. 10,wherein the .Van de- Graaff generator 70 and the column 74 depicted inFIG. 10 is summarized as the particle accelerator circuitry 26, andwherein the accelerator circuitry 26 is coupled to the vacuum ortime-of-flight chamber 20 by the sensitive particle detector 27. As maybe seen in FIG. 2, therefore, the chamber 20 contains an impact plate ortarget 22 disposed between the grid 23 and the evacuation port 18, andspaced a suitable distance from the collector electrode assembly 25which is located in the other end of the chamber 20. The grid 23 isgrounded, as hereinbefore stated, and the target 22 is accordinglycoupled to a suitable high voltage supply 21, whereby impactingparticles will produce a suitable indicating signal which is preferablyenhanced by the target amplifier 24 and forwarded to the controlcircuitry 96 depictedin FIG. 10.

As suggested in FIG. 2, the collector assembly 25 is preferably composedof a flat ion collector plate or electrode 25A, which is disposed sothat a particle issuing from the accelerator circuitry 26 will passthrough a small port 258 as it travels to the target 22 by way of apreselected path 20A. The electrode 25A is preferably surrounded by agrounded shield 25B, and the electrode 25A is preferably coupled to acollector amplifier 29 which also transmits a signal to the controlcircuitry 96 in FIG. 10.

Referring again to FIG. 1, it will be seen that the sequence begins whena particle exits from the accelerator 26 which is found by the timeinterval selector and generator 79 (see FIG. 10) to have a transit timewhich age on the particle deflector 82 is removed, and the particle tobe analyzed is permitted to enter the chamber 20.

Just before the particle reaches the sensitive particle detector 27,however, the proportional delay generator 79 produces trigger pulse 798to initiate the sweep of the oscilloscope in the display circuit 94,whereby signal amplitude and transit time will be presented to thecamera 95 as the particle traverses the detector 27. This informationprovides the basis for calculating the various particle parameters(velocity, mass and radius).

The particle then passes through the pinhole aperture or port 258 in thecollector electrode 25A as it continues along the preselected path 20Ato the target 22. Just before the particle impacts on the target 22,however (and presumably just as it reaches location X in the chamber 90depicted in FIG. 10), the delay generator 79 initiates the secondoscilloscope sweep which will present the target impact signal (and theion charge and transit time information) to the camera 95. Ashereinbefore stated, the target signal which is provided by the targetamplifier 24 is essentially a step function for particle impacts ofgreater than a certain magnitude (i.e., about 10 km/s), whereas thesignal provided by the collector amplifier 29 tends to rise in staircasefashion. Either or both waveforms may be differentiated, however, toproduce sharp pulses (spikes") either at impact on the target 22 or uponarrival of each ion group at the collector electrode 25A.

Referring again to FIG. 2, it will be noted that the impacting particlewill not depart substantially from the transit path 20A, and thus thetarget 22 may be relatively small in diameter. On the other hand, thethermal energy of the resulting ions tends to cause the ions to divergeas represented in FIG. 2 by the diverging arrows emanating from thetarget 22. Accordingly, a relatively larger collector electrode 25A isrequired to collect an adequate number of these ions. In order tofurther provide that an adequate number of ions be collected, the sideof the shield 25C which confronts the active or receiving side of thecollector electrode 25A is preferably made from screening cloth or thelike which has a reasonably high transmittance.

It may be noted by those having particular skill and discernment in thisart that the apparatus depicted in FIG. 2 may, on occasion, tend to havea relatively low signal-to-noise ratio. Contrary to expectation, thisdisadvantage is not due to any inherent defect in the apparatus, but iscaused by the small size of the particles which are delivered to thechamber 20 by the accelerator assembly 26, or which are encountered bythe target 2 (see FIG. 1), when the basic components of the system areemployed in outer space to analyze the constituency of cosmic dust orthe like.

Referring now to FIG. 3, there may be seen a functional representationof apparatus which is substantially the same in concept as that depictedin FIGS. 2 and 10, buy which has been modified to reduce or eliminatethe problem of the low signal-to-noise ratio. In particular, it will benoted that an ion detector assembly has been incorporated in conjunctionwith the ion collector assembly 35 and that this has required that theparticle be introduced into the chamber 30 on an off-axis trajectory.Thus, the particle is required to impact on the target 32 at an angle(perhaps 9 degrees) relative to the axis of the chamber 30, and this inturn requires that a smaller collecter plate or electrode 35A beemployed, although the trajectories of the ions emanating from thetarget 32 will preferably remain the same.

Referring again to FIG. 3, it will be seen that the accelerator 36 andsensitive particle detector 37 are cou pled to the chamber 30 at anoff-axis manner. as hereinbefore stated, since the collector assembly 35and ion detector assembly 15 are necessarily positioned in the chamber30 at a preselected spacing in the chamber 30 from the target 32 andgrid 33. The target 32 is positioned adjacent the port 19 which iscoupled to an auxiliary vacuum pump (not depicted) and is further cou'pled between a suitable voltage supply 31 and the input of the targetamplifier 34. The collector electrode 35A is also disposed in a groundedgrid 35C, as hereinbefore explained, and is coupled to a collectoramplifier 39.

Referring now to FIG. 4, there may be seen a more detailedrepresentation, which is partly pictorial and partly functional, of theion detector assembly 15 depicted in FIG. 3. In particular, the iondetector assembly l5 proper may be seen to be composed of a conventionalend-window photomultiplier tube 49, which is energized in a conventionalmanner by a suitable high voltage supply 52, and which produces anoutput signal which is enhanced by an amplifier 50 before being appliedto the vertical amplifier (not depicted) in the display electronics 94represented in FIGv 10. The sensitive portion of the ion detectorassembly 15 may be seen in FIG. 4 to be composed of a suitable phosphor46 (such as a thallium-activated crystal formed of sodium iodide or thelike) which has a relatively thin sheet or coating of metal 45(preferably a 1,000 Angstrom aluminum film) on the side of the phosphor46 which faces the collector electrode 40. The phosphor 46 is preferablyoptically coupled to the window end of the photomultiplier tube 49 whichcontains the photocathode (not depicted) by a truncated light pipe 47 orother suitable means, whereby all of the scintillations which occur inthe larger diameter phosphor 46 are nevertheless visible" to thephotocathode in the smaller diameter window of the photomultiplier tube49. The phosphor 46 is preferably secured to the receiving side or faceof the light pipe 47 by an annular rim bracket or clamp 48, which ispreferably formed of some electrically conductive material whereby itmay also be employed to couple a suitable high voltage supply to themetal film or sheet 45 on the phosphor 46.

Referring to FIG. 3, there will be noted a ring-type or annularaccelerating electrode 43 which is positioned concentrically about theaxis of the chamber 30 and spaced appropriately between the ion detectorassembly l5 and the ion collector assembly 35. Referring again to FIG.4, the collector assembly 35 depicted in FIG. 3 may be seen to include aslotted collector electrode 40 disposed within a shield assembly whichis composed of a grounded annular grid bracket 41 which mounts a highlypenetrable shield 42 on the target-side of the collector electrode 40and which also supports a second penetrable shield 53 on the so-calledrearward" side of the electrode 40.

The purpose of the ion collector assembly 35 and ion detector assembly15 depicted in FIGv 3 is to convert the ion to be detected into one ormore secondary electrons, to accelerate these electrons to increasetheir energy, to thereafter convert a portion of such energy intoscintillations of functionally proportional magnitude,

and then to produce voltage pulses which are functionpressor grid 42 tothe collector electrode 40 to activate the collector amplifier 39 ashereinbefore explained. Some of the ions will, however, pass through theslots 54 in the collector electrode 40, and, after traversing the exitgrid 53, these ions will enter" the high voltage field which exists inthe region between the exit screen 53 and the accelerator electrode 43.

As hereinbefore stated, the annular accelerator electrode 43 isconcentrically arranged relative to the axis of the ion beam emanatingfrom the target 32, and thus the accelerator electrode 43 will functionas a secondary electron source as well as an ion accelerator electrode43. Moreover, it will be noted that the accelerator electrode 43 ispreferably provided with an ion impact surface 43A which is positionedat about 45 degrees relative to the metal film 45 on the receiving faceof the phosphor 46, in order to enhance the production of secondaryelectrons in response to bombarding ions which pass through the slots 54in the collector electrode 40. These secondary electrons from theaccelerating electrode 43 will then be drawn into the phosphor 46 by thedifference between the potential established on the electrode 43 by thehigh voltage supply 44, and the potential or charge established on thealuminum film 45 by the power supply 51. Since the aluminum coating orfilm 45 is preferably very thin, the electrons will penetrate the film45 and be captured in the phosphor 46 without substantial loss ofenergy, and each electron capture will result in the production of anumber of photons in the phosphor 46, the number of which isfunctionally related to the energy of the captured electron. The purposeof the low noise photomultiplier tube 49 is to generate a pulse for eachset of scintillations or photons thus produced which is functionallyrelated in amplitude to the number of photons present, and thus theamplitude of the pulse is proportional to the energy of the capturedelectron which created the photons. The output signal from thephotomultiplier tube 49 is then buffered with a unity gain amplifier 50before being presented to the vertical amplifier (not depicted) of theoscilloscope (also not depicted) in the display circuitry 94representedin FIG. 10.

Referring now to FIG. 5, there may be seen a simplified functionaloverall representation of an exemplary embodiment of the presentinvention as adapted for use in outer space. More particularly, there isindicatedthe major components of the system in a manner so as to suggesttheir contribution to the operation of the system. Accordingly, when amicrometeorite strikes the target electrode 60, this will causepositively-charged ions to emanate from the target 60, whereupon anegative charge will appear on thetarget electrode 60. This negativecharge is split betweentwo charge-sensitive amplifiers 62 and 63, andtheir outputs are accordingly fed to a signal conditioning circuit 64,which is composed of target amplifiers and discriminators (not depictedin FIG. which operate to amplify and determine the approximate level ofthe input charge signal. As will hereinafter beapparent, this signalmust lie withon one of five decade ranges, and thus the conditioningcircuit 64 will also preferably include logic for providing a three-lineoutput signal to identifythe particular decade range. This signal, inturn, is transmitted to the collector range switch assembly 67 whichoperates to set the appropriate signal gain in the collector electronics67. It will also be noted that the three-line output signal from thetarget signal conditioning circuit 64 is also transmitted to the datamemory circuit 69 for purposes of storage.

As will hereinafter be explained in detail, the function of thethree-line output signal is to condition the switch assembly 67 so thatthe amplitude of the output signal will not exceed the acceptance levelof the analog-to-digital converter 68. The ion charge which emanatesfrom the target electrode 60, travels to the collector electrode 61 ashereinbefore described, whereupon the resulting signal is also splitbetween two chargesensitive amplifiers 65 and 66 to produce thestaircaselike output signal which is presented to the amplifiers (notshown in FIG. 5) in the collector range switch assembly 67. Accordingly,it will be apparent that the two charge-sensitive amplifiers 65 and 66as well as the amplifiers in the switch assembly 67 are effectivelyunderthe control of the three line output signal from the target 60.

Referring again to FIG. 5, when the most sensitive of the discriminatorsin the conditioning electronics 64 is tripped, a signal will betransmitted to the timing and control logic circuit 59 which is includedfor the purpose of establishing a time base for sampling the output fromthe collector electrode 61 at eight different times, to provide timingfor the converter 68 of these eight sampled signals, and to provideother control functions for the entire period of signal acquisition,conversion, and transfer of acquired data to the telemetering circuit(not depicted) in the space-craft. Accordingly, experiment timing may beprovided by a l0-million pulsesper-second clock circuit 58 as willhereinafter be explained.

More particularly, the staircase-like signal produced by the collectorelectrode 61 is preferably sampled an appropriate number of times (i.e.,eight) selected arbitrarily during a preselected period (preferably25.64:. sec.) following particle impact on the target electrode 60.These eight sampled points are then stored as analog voltages for laterconversion to digital form, and after the initial 25.6;1. sec. signalacquisition period, the digital-to-analog converter 57 begins producingan output signal from a slow clock (not depicted in FIG. 5) in theconverter circuit 57. Thus, the output signal from the converter 57 is astaircase ramp voltage signal which is compared to the aforementionedanalog-type signals stored in the memory circuit 69.

When the clockin the converter 57 reaches its maximum count, all storedanalog data will have been converted and stored in the appropriatesection or stage of the memory circuit 69. In addition, attainment ofthe maximum count from the clock in the converter 57 will freeze thesystem in a holding mode and the timing and control logic 59 willsimultaneously produce a data ready" signal to condition the spacecrafttelemetry (not depicted). However, actual transfer of data will notoccur until a "data enable signal is produced during data transfer andthe application of a preselected number of telemetry pulses to thememory circuit 69. Removal of the data enable signal signifiescompletion of data transfer out of the memory circuit 69 to thespacecraft telemetry, whereupon the system reverts to its standby modeto await the arrival of the next impinging micrometeorite. 1

Referring again to FIG. 5, it is the function or purpose of thecollector range switch assembly 67 to convert the charge appearing atthe collector electrode 61 to an analog voltage output. Thus, the gainof each of the collector amplifiers is preferably set at one ofacorresponding number of ranges, depending on the charge previouslysensed at the target, to accommodate the relatively wide dynamic inputrange. The charge is split between the two identical charge-sensitivepreamplifiers 65 and 66, and the splitting ratio for these amplifiers 65and 66 is preferably ten times the ratio for the collector amplifiers.

Referring now to FIG. 6, there may be seen a more detailed schematicrepresentation of one embodiment of apparatus suitable for use as thetiming and control logic circuitry 59 generally depicted in FIG. 5. Inparticular, it will be noted that when a start clock" pulse 101 isreceived, clock pulses 102 will then step the 9-bit synchronous counterwhich is composed of the nine flip-flop circuits 120-128 and the sixteengates 130-145. Accordingly, as will also be noted, the states of the9-bit counter are decoded to provide a variety of functions. The eightgates 150-157 each have eight inputs (for simplicity only one beingillustrated) which are discretionary wired to the outputs of the 9-bitcounter to independently set the gate companion latches (see gates160-175) at any time from 0, which is the arrival of the start clockpulse 101, to whatever preselected time may be deemed appropriate. Inthis respect, it will be noted that the gate companion latches, in turn,provide hold signals 107 of the each of the eight analog converters.

When the 9-bit counter circuit has received 256 pulses 102, the lastflip-flop circuit 128 will go to its true state, and theanalog-to-digital converter 68 will begin its scan cycle. When theflip-flop 128 changes state, however, this causes the two gates 118 and119 to change state and this, in turn, causes the switching circuitcomposed of the gates 180-186 to change the clock pulses 102 from the9-bit counter circuit to a divider circuit composed of the six flip-flopcircuits 190-195. The 9-bit counter is now counted down to the all onesstate, at which time the 9-bit counter is locked until a data enablesignal 105 is received from the spacecraft data system (not depicted)which indicates that a new measurement can be taken. Accordingly, itwill be seen that the 9-bit counter serves four primary purposes: (1)generate timing signals to control subsequent events, (2) provide holdcommand signals to the analog storage circuits, (3) provide timingsignals to control the analog-to-digital converter (ADC) 68 in itsprocessing of the data held in the storage circuits, and (4) freezingthe system until all digitized data are transferred to the spacecraftdata system and a data enable signal 105 is received.

Referring now to FIG. 7, it may be seen that the ADC load signal 200 andthe ADC bit 1-8 signals 201-208 depicted in FIG. 6, are coupled in FIG.7 to the analog gate circuits 210-218 respectively. These gate circuits210-218, in turn, are coupled to an appropriate plurality of 4-bitparallel entry shift registers 220-236, and are also coupled to an 8-bitdigital-to-analog converter 240.

During the analog-to-digital cycle, the states of the first eight stagesof the 9-bit synchronous counter (in FIG. 6) are such that they areaccordingly connected to registers 210-227, registers 229-236, and tothe digital-to-analog converter 240. Accordingly, a ramp voltage 241will be generated by the amplifier 242, which starts at zero, but whichincreases in a step-wise manner until it reaches the 255th step. Eachstep lasts a preselected time interval which is determined by thecapacity of the 9-bit synchronous counter and divider circuit depictedin FIG. 6.

The inverted outputs of the analog storage circuits 250A-258A are eachconnected to the enable inputs of two of the 4-bit registers 220-236.The ramp, however, is simultaneously connected to all of the analogstorage circuits 250-258 as will hereinafter be seen in FIG. 8.

It will be apparent that all outputs of gates 250-258 will initially bein the low state because of a high state at the enable inputs of the4-bit shift registers 220-236. The load pulse 200 from theanalog-to-digital converter 68, which occurs during each update of the9-bit counter in FIG. 6, causes the successive counts to be transferredinto each of the 4-bit registers 220-236 except for register 228. Inother words, the registers will continuously track the 9-bit counter,and as the ramp voltage 241 equals the capacitor voltage in each of theanalog storage circuits 250-258, its output will go high, therebyremoving the enabling gate to its companion pair of 4-bit shiftregisters 220-236, and thereby freezing therein the previous count inthe 9-bit counter. Accordingly, this count will be the digitized valueof the voltage which is in the analog storage circuits 250-258.

This process will be seen to continue until the count in the 9-bitcounter reaches 255, and thus all eight of the inputs to the analogstorage circuits 250-258 will be scanned. However, the range bits in thecircuits 261-263 will be stored in the 4-bit shift register 258 whichhas heretofore been skipped in the process.

As hereinbefore explained, when the 9-bit counter in FIG. 6 is full(reaches the all ones state), the data ready signal 103 will be sent tothe spacecraft data system (not depicted) to indicate that theexperimental data is ready for readout. Readout, of course, commencesupon receipt of both the appropriate clock signal 260 from thespacecraft telemetry circuits (not depicted) and the data enable signal105. Only sixty-eight clock pulses 260 will now be required to causereadout of all data stored in the depicted eight-channel system.Further, the data enable signal 105 will bracket the readout clockpulses 260, and will be used internally to reset the 9-bit counter andcontrol latches. After the last bit of data has been shifted out by thespacecraft telemetry circuits, the data enable signal 105 will drop tocause the circuitry depicted in FIG. 7 to revert to its low-powerstandby mode.

Referring now to FIG. 8, there may be seen a simplified schematicrepresentation of a circuit which may be employed as the analog storagecircuits 251A-258A suggested in FIG. 7. For simplicity, however, it willbe noted that only one analog storage circuit 251A is depicted indetail, since all eight channels are identical.

Referring again to FIG. 8, it will be seen that in the hold mode(wherein the field effect transistor coupled to input A is off, andwherein the other field effect transistor coupled to input 161A is on),the voltage across the capacitor 251AA remains within 0.1 percent of itstracked value for a preselected time interval which is the periodrequired to perform the analog-todigital conversion on all held samples.The field effect transistor 251BB is turned on at all times except whena measurement is being made. After a start clock pulse 101 is generatedby the collector circuitry 67, the comparator reset voltage 107 will goto lowthereby blocking or turning off the transistor 25188, and thiswill occur after the expiration of a preselected period beginning withthe start of the start clock signal 101.

The two field effect transistor switches coupled to unputs 160A and 161Aare controlled by timing circuits in the logic circuitry 69 ashereinbefore explained. Accordingly, the tracking turn-off time for eachof the eight analog converter circuits 251A-258A will be controlledindependently of the others.

It will be further noted that the ADC enable voltage 209 is held low fora different longer preselected time interval following the start of thestart clock signal 101. This clamps the ramp voltage 241 at zero toprevent spurious outputs from appearing from any of the analog convertercircuits 25lA-258A. Further, it is expected that this different longertime interval during which the ADC enable voltage 209 is held low, willbe substantially longer than the period during which all ions ofinterest from the target electrode 60 are collected.

Referring now to FIGS; 9A and B, there may be seen a simplified butdetailed schematic representation of circuitry which may be employed tofunction as the collector circuitry 67 and charge-sensitive amplifiers65-66 depicted in FIG. 5. Referring to FIG. 9A in detail, it will benoted that the circuitry illustrated therein is conventionally suitablefor use as the two chargesensitive amplifiers 65 and 66, and that bothsuch preamplifiers 65 and 66 are therefore identical in design andfunction. However, it will also be noted that a pair of grids 61A andBare also preferably included with the collector electrode 61 forreasons which will be readily apparent to those with experience in thisart.

As hereinbefore stated, charge splitting is used to divide the chargereceived by the collector electrode 61 between the two identicalcharge-sensitive preamplifiers depicted in detail in FIG. 9A. A suitablesplitting ratio has been found to be 1000 to 1, as compared with theIOO-to-l ratio which is preferably employed for the two charge-sensitivepreamplifiers 62 and 63 employed with the target electrode 60.

The balance of the collector circuitry 67 depicted generally in FIG. 5,may be seen in FIGS. 9A and 98 to be composed of post amplifiers andgain control switches of conventional design. It should be furthernoted, however, that the selection of the particular one of the twopreamplifiers 65 and 66 depends on the state of the voltage at the inputterminals 673 and 67C, in order to provide the gains desired for thepost amplifiers depicted in FIG. 9B. Accordingly, FIG. 9A is directed tothe illustration of examplary circuitry suitable for the gain controlswitch portion of the collector circuitry 67 in FIG. 5, as well as thetwo charge-sensitive amplifiers 65 and 66. On the other hand, FIG. 9B isdirected primarily to circuitry which is conventional but nonethelesssuitable for use as the post amplifier portion of the collector circuit67.

When a plasma in thermal equilibrium at absolute temperature T containsseveral species of atoms and ions, the degree of ionization of eachspecies whose ionization energy is E, is related to T according to Sahasequation (cf. Sutton and Sherman 1965): n,. lh'n a], "I'm p 'I' where:n,, m, and n,,' are the numbers per unit volume of electrons, ions ofspecies 5, and neutral atoms of species s, respectively; C is aconstant; m", n is the ratio u,"

/u,," in which u, and u,, are respectively the internal partitionfunctions of ions and neutrals of species s; and k is Boltzmannsconstant. The Saha equation shows that the fractional ionization of agiven species of atom depends on the ratio of the ionization energy ofthat species to the mean thermal energy of the plasma. For reasonablylow plasma temperatures, only a small fraction of the atoms are ionized(except for those with very low ionization potentials). In order to usethe impact ionization effect to reliably determine the relativeabundance of the elements in cosmic dust and meteoroids, a measure ofthe plasma temperature must be obtained. If the impact target containstwo known elements in known concentration, the measurement of therelative ion signals of these two elements can be reduced to plasmatemperature by taking the ratio of the Saha equation. v

Further modifications and alternative embodiments will be apparent tothose skilled in the art in view of this description, and, accordingly,the foregoing specification is considered to be illustrative only.

We claim:

1. Apparatus for investigating the character of particulate mattertraveling at high energies, comprising a target electrode means havingtwo known elements in known concentration, responsive to random impactof said particulate matter to emit a mixture of electrons and ions oftarget material and particulate matter,

grid means for suppressing said electrons from said target electrode,

time-of-flight means for counting said ions from elements of the targetand the ions from the particulate matter as a function of theircharge-to-mass ratio, and

register means for deriving a functional indication of the rate ofoccurrence of each of said ions of a particular mass whereby withutilization of the Saha equivalent equation the plasma temperature isascertainable and thereby the true relative abundance of the elements ofthe particulate matter.

2. The apparatus described in claim 1, wherein said time-of-flight meansfurther includes a collector electrode arranged a predetermined distancefrom said target electrode and grid means for receiving said ions, and

1 means for establishing a field-free region between said collectorelectrode and said grid means.

3. Apparatus for investigating the character of particulate mattertraveling at high energies, comprising a target electrode having twoknown elements in known concentration arranged for impact with saidparticulate matter,

electron suppression means adjacent said target electrode,

a collector electrode spaced a preselected distance from saidsuppression means and cooperating with said target electrode to define afield-free region therebetween,

timing means for determining the time-of-flight of ions from elements ofthe target and the ions from the particulate matter of said particulatematter traveling to said collector electrode after impact with saidtarget electrode, and

display means responsive to said timing means for providing a recordablemass-dependent indication of the number of ions traversing said distanceduring a determined time interval whereby by utilization of the Sahaequivalent equation the plasma temperature is ascertainable and therebythe true relative abundance of the elements of the particulate matter.4. The apparatus described in claim 3, wherein said collector electrodefurther includes a phosphor for deriving scintillations in functionalrelationship to said ions emanating from said target electrode, andphotoelectric means responsive to said scintillations for derivingelectrical signals functionally related in magnitude to saidscintillations. 5. The collector electrode described in claim 4, furtherincluding electron generating means responsive to said traversing ionsfor producing functionally related secondary electrons, and acceleratingmeans for receiving and accelerating said secondary electrons into saidphosphor. 6. The apparatus described in claim 5, further includingindicating means for deriving from said electrical signals a recordableindication functionally related to the mass of said particulate matterimpacting on said target electrode and producing said traversing ions.7. Apparatus for investigating the character of particulate mattertraveling at high energies, comprising a target electrode of having twoknown elements in known concentration arranged in a high vacuumatmosphere to emit ions of said target material and said particulatematter in response to impact by said particulate matter, targetamplifier means for generating a first timing signal functionallyrelated to said impact of said particulate matter, and a sensitive iondetector spaced a predetermined distance from said target electrode insaid high vacuum atmosphere for receiving said ions emitted by saidtarget electrode and for deriving therefrom a second timing signalfunctionally related to the time-of-flight of said ions from said targetelectrode and an indicating signal representing the number of saidimpacting ions,

from elements of the target and the ions from the particulate matter,

whereby by utilization of the Saha equivalent equation the plasmatemperature is ascertainable and thereby the true relative abundance ofthe ele ments of the particulate matter.

8. The apparatus described in claim 7, wherein said ion detectorincludes accelerating means for increasing the energy of said ions tovelocities which are dependent substantially only on the mass of saidions.

9. The apparatus described in claim 8, wherein said ion detector furtherincludes a collector electrode means spaced a predetermined distancefrom said target electrode, and

means for establishing a field-free condition between said target andcollector electrodes.

10. The apparatus described in claim 9, further including indicatingmeans connected with said collector electrode for deriving an observablesignal which is functionally indicative of the number and atomic mass ofsaid ions.

11. The apparatus described in claim 7, wherein said ion detectorincludes an ion collector assembly arranged a predetermined distancefrom said target electrode,

an ion detector assembly in conjunction with said ion collectorassembly, and

means directing the particulate matter to strike the target at an anglethereto.

1. Apparatus for investigating the character of particulate mattertraveling at high energies, comprising a target electrode means havingtwo known elements in known concentration, responsive to random impactof said particulate matter to emit a mixture of electrons and ions oftarget material and particulate matter, grid means for suppressing saidelectrons from said target electrode, time-of-flight means for countingsaid ions from elements of the target and the ions from the particulatematter as a function of their charge-to-mass ratio, and register meansfor deriving a functional indication of the rate of occurrence of eachof said ions of a particular mass whereby with utilization of the Sahaequivalent equation the plasma temperature is ascertainable and therebythe true relative abundance of the elements of the particulate matter.2. The apparatus described in claim 1, wherein said time-of-flight meansfurther includes a collector electrode arranged a predetermined distancefrom said target electrode and grid means for receiving said ions, andmeans for establishing a field-free region between said collectorelectrode and said grid means.
 3. Apparatus for investigating thecharacter of particulate matter traveling at high energies, comprising atarget electrode having two known elements in known concentrationarranged for impact with said particulate matter, electron suppressionmeans adjacent said target electrode, a collector electrode spaced apreselected distance from said suppression means and cooperating withsaid target electrode to define a field-free region therebetween, timingmeans for determining the time-of-flight of ions from elements of thetarget and the ions from the particulate matter of said particulatematter traveling to said collector electrode after impact with saidtarget electrode, and display means responsive to said timing means forproviding a recordable mass-dependent indication of the number of ionstraversing said distance during a determined time interval whereby byutilization of the Saha equivalent equation the plasma temperature isascertainable and thereby the true relative abundance of the elements ofthe particulate matter.
 4. The apparatus described in claim 3, whereinsaid collector electrode further includes a phosphor for derivingscintillations in functional relationship to said ions emanating fromsaid target electrode, and photoelectric means responsive to saidscintillations for deriving electrical signals functionally related inmagnitude to said scintillations.
 5. The cOllector electrode describedin claim 4, further including electron generating means responsive tosaid traversing ions for producing functionally related secondaryelectrons, and accelerating means for receiving and accelerating saidsecondary electrons into said phosphor.
 6. The apparatus described inclaim 5, further including indicating means for deriving from saidelectrical signals a recordable indication functionally related to themass of said particulate matter impacting on said target electrode andproducing said traversing ions.
 7. Apparatus for investigating thecharacter of particulate matter traveling at high energies, comprising atarget electrode of having two known elements in known concentrationarranged in a high vacuum atmosphere to emit ions of said targetmaterial and said particulate matter in response to impact by saidparticulate matter, target amplifier means for generating a first timingsignal functionally related to said impact of said particulate matter,and a sensitive ion detector spaced a predetermined distance from saidtarget electrode in said high vacuum atmosphere for receiving said ionsemitted by said target electrode and for deriving therefrom a secondtiming signal functionally related to the time-of-flight of said ionsfrom said target electrode and an indicating signal representing thenumber of said impacting ions, from elements of the target and the ionsfrom the particulate matter, whereby by utilization of the Sahaequivalent equation the plasma temperature is ascertainable and therebythe true relative abundance of the elements of the particulate matter.8. The apparatus described in claim 7, wherein said ion detectorincludes accelerating means for increasing the energy of said ions tovelocities which are dependent substantially only on the mass of saidions.
 9. The apparatus described in claim 8, wherein said ion detectorfurther includes a collector electrode means spaced a predetermineddistance from said target electrode, and means for establishing afield-free condition between said target and collector electrodes. 10.The apparatus described in claim 9, further including indicating meansconnected with said collector electrode for deriving an observablesignal which is functionally indicative of the number and atomic mass ofsaid ions.
 11. The apparatus described in claim 7, wherein said iondetector includes an ion collector assembly arranged a predetermineddistance from said target electrode, an ion detector assembly inconjunction with said ion collector assembly, and means directing theparticulate matter to strike the target at an angle thereto.